The radio frequency range of the electromagnetic spectrum is used for transmitting information signals between transmitters and receivers in many different types of communication systems, and is divided into standard frequency ranges which are assigned to the different types of systems to prevent interference between transmissions. Ideally, each transmitter should have its own assigned frequency range, but while the total available radio frequency range is intrinsically limited, the number of systems is not. Instead, the assigned frequency range must somehow be shared by all the transmitters of the system. Frequently the assigned frequency range is itself divided up into a number of transmission channels, but even so there are usually more transmitters than transmission channels, so that the transmission channels themselves must be shared.
A variety of communications systems are presently available which include a number of transmitters and a number of receivers, each transmitter able to transmit information signals over any one of several transmission channels, and each receiver able to receive such information signals transmitted over any of these transmission channels. It is usually intended that each transmitter will establish a transmission channel only with one respective receiver. An example of such a communications system is a cordless telephone system, where a particular frequency range has been assigned by the Federal Communications Commission to all cordless telephones within the system. Needless to say, the number of cordless telephones already greatly exceeds the number of available transmission channels.
Therefore, methods have been developed for sharing the available frequency range among the cordless telephones within the system so as to minimize interference. This interference could be in the form of noise from other transmissions or from actual conversations picked up from nearby cordless telephones, each operating over the same transmission channel. Each transmission channel generally includes an audio signal transmission band over which the audio signals will be transmitted once the transmission channel has been established between a particular transmitter and a particular receiver. When no transmitter is transmitting over a particular transmission channel, it is known that the electric field intensity within the corresponding frequency range wilI be less than a predetermined level. According to one method, then, each transmitter sequentially scans the transmission channels and detects the received electric field intensity at the corresponding frequency ranges to determine whether any of the transmission channels are unoccupied, and selects an unoccupied one for transmission. Once a particular transmission channel has been selected, the audio signal transmission band thereof is used for transmitting an identifying code, uniquely identifying the transmitter to its respective receiver. This identifying code is used by the receiver to establish the transmission channel between itself and the transmitter. Each transmitter/receiver pair has identifying codes uniquely assigned to it by which each component may identify signals transmitted by the other.
Meanwhile, the receiver carries out a conventional polling operation whereby it checks each of the transmission channels to see if the correct identifying code is present. When this is detected, the receiver stops the polling operation and establishes the transmission channel over which it received the correct identifying code between itself and the transmitter. Thereafter, the receiver is adapted to receive the audio signals from the transmitter over this particular transmission channel.
However, while the audio signal transmission band of the particular transmission channel may be used to transmit the identifying code before the transmission channel is established between the transmitter and receiver, it can no longer be so used once the transmission channel is established and an actual audio signal is being transmitted. In such case, there would be interference between the identifying code and the actual audio signal. On the other hand, if the identifying code is no longer transmitted, there is no way to continue to check that the signal is still being received from the correct transmitter. It would be highly advantageous to continue to transmit the identifying code after the transmission channel is established.
To remove this defect, it has been proposed to mix the identifying code with the actual audio signal by time division multiplexing, so that the identifying code may be repetitively transmitted to the receiver. The receiver discriminates between the actual audio signal and the identifying code on a time basis. However, this requires that the actual audio signal be periodically interrupted to allow the transmission of the identifying code, which can lead to objectionable results when the audio signal is transformed into sound.
Another proposal is to transmit the identifying code over a separate frequency band, i.e. frequency division multiplexing. However, if this separate frequency band has a bandwidth equivalent to the standard audio signal transmission band, this will rapidly use up the available bandwidth, reducing the number of available transmission channels. If, on the other hand, only a narrow bandwidth is used for the transmission of the identifying code, then the bit transmission rate for the identifying code is reduced, so that it takes a relatively long time to transmit the identifying signal and thereby to establish the transmission channel between the transmitter and the receiver.